1,5-Elektrocyclisierungen – ein wichtiges Prinzip der Heterocyclen-Chemie

A Synthetic Route Toward Tetrazoles: The Thermolysis of Geminal Diazides

A new synthetic route toward the tetrazole core is described, which is based on a general fragmentation pattern that was found in a range of compounds featuring geminal diazido units. Through a simple two-step procedure, the synthesis of structurally diverse target compounds containing a tetrazole, such as tetrazoloquinoxalinones, benzoylaryltetrazoles, tetrazolotriazinones, and tetrazoloazepinones, was easily accomplished, starting from broadly accessible substrates (i.e., oxindoles, diarylethanones, pyrazolones, and phenanthrols). The initial oxidative diazidation reaction with iodine and sodium azide under mild conditions is followed by the thermal fragmentation under microwave irradiation, leading to the tetrazole products. Noteworthy, an experimental solution is presented in which the potentially hazardous diazide intermediates are not isolated and the concentration of crude reaction mixtures containing diazides is not required to achieve the tetrazoles in good yields.

Gold-Catalyzed Bicyclic Annulations of 2-Alkynylbenzaldehydes with Vinyldiazo Carbonyls that Serve as Five-atom Building Units

This work reports gold-catalyzed bicyclic annulations of 2-alkynyl-1-carbonylbenzenes with vinyldiazo ketones that serve as five-atom building units. The importance of these reactions is to access 4,5-dihydro-benzo[g]indazoles, which form the structural cores of various bioactive molecules. According to our mechanistic analysis, we postulate initial [5+4]-cycloadditions between benzopyrilium intermediates and vinyldiazo ketones, followed by 6-π-electrocyclizations to achieve the excellent stereoselectivity.

Cyclopentene Annulations of Alkene Radical Cations with Vinyl Diazo Species Using Photocatalysis

A direct (3+2) cycloaddition between alkenes and vinyl diazo reagents using either Cr or Ru photocatalysis is described. The intermediacy of a radical cation species enables a nucleophilic interception by vinyl diazo compounds, a departure from their traditional electrophilic behavior. A variety of cyclopentenes are synthesized using this method, and experimental insights implicate a direct cycloaddition instead of a cyclopropanation/rearrangement process.

Ring closure reactions of 2,6-diazaheptatrienyl metal compounds: synthesis of 3-aminoindole derivatives and 14-membered macrocyclic dimers

2,6-Diazaheptatrienyl metal compounds 6(-)K(+) are easily accessible from the corresponding diimines 6 by deprotonation using KO-t-Bu as base. According to quantum chemical calculations, they are, in comparison to other isomeric species with nitrogen atoms in other positions, highly reactive intermediates, which undergo in dilute solution at 50 °C ring closure reactions to form 3-aminoindole derivatives 8/10. In contrast, in more concentrated solution at room temperature, the formation of 14-membered macrocyclces 13 as a result of formal dimerization is observed. The 3-aminoindole derivatives obtained in this work possess rare substitution patterns, and the macrocyclic compounds are essentially unknown. Two-fold vinylogous derivatives 7 give rise to tricyclic systems with δ-carboline backbone 12. The experimental results are interpreted using high-level DFT calculations with regard to the possible reaction mechanism and the nature of the transition state of the five-membered ring formation. The molecular structures in the solid state of all types of compounds were elucidated by X-ray diffraction.

Roots: From carbenes to allenes and coordination polymers : Ever present never twice the same

The purpose of this Feature Article is to demonstrate that recognizing the similarities in different areas of chemistry allows the prediction of potential results in related fields. For instance, during our investigations of 2,2-diethoxyvinylidene-triphenylphosphorane we became interested in 2,2-diethoxydiazoethene. In order to obtain diazoethenes, we studied vinyl-diazonium salts and geminal vinyl-diazides as precursors. In the course of these investigations, we realized their synthetic potential to produce, via substituent-dependent 1,5-, 3,5-, or 1,5'-cyclization, a whole variety of heterocycles. However, more importantly, we became familiar with the chemistry of carbenes, which prompted an investigation of the carbene-like character of push-pull substituted allenes. Due to the ambiphilicity of their central carbon atom, they readily dimerized. Consequently, our strong interests in push-pull substituted allenes drew our attention to tetradonor substituted allenes, and as a result, we employed tetraethoxyallene as a synthetic equivalent to the fictitious malonic ester 1,1-/1,3-dianion synthon. This concept led to the synthesis of heterocumulenes and to the transallenation reaction to give allenecarboxanilides, which was further developed as a cumuhomologation for the synthesis of butatrienes via haloallenes from propargyl alcohols. The Diels-Alder reaction and intramolecular domino cyclization of multi-functional allenecarboxanilides yielded complex fused heteroarenes. Finally, the 1,5-cyclization of vinyl-azides, reported earlier, provided tetrazolylidene ligands, triggering our interest in supramolecular coordination chemistry, for example, the synthesis of one-, two- and three-dimensional coordination polymers.

Improved Reaction and Activation Energies of [4+2] Cycloadditions, [3,3] Sigmatropic Rearrangements and Electrocyclizations with the Spin-Component-Scaled MP2 Method

A new quantum mechanical scheme to calculate electronic correlation energies, spin-component-scaled MP2, was tested as a tool to predict reaction energies and barriers in computational organic chemistry. Three common pericyclic reactions with known unsatisfactory MP2 descriptions were reinvestigated with the modified MP2 approach, in which the parallel and anti-parallel spin components of the correlation energy are scaled separately. The SCS-MP2 calculated reaction and activation energies of nine Diels-Alder reactions, four [3,3] sigmatropic rearrangements, and ten electrocyclization reactions are compared to those of the MP2, B3 LYP, QCISD(T), and G3 methods. For each, the SCS-MP2 results are in excellent agreement with the experimental data and compare far more favorably to G3 than both MP2 and B3 LYP. Careful evaluation of the effect of the size of the atomic orbital (AO) basis set shows that the larger expansions improve the agreement with experiment for the SCS-MP2 method, while they get worse for both MP2 and B3 LYP.